Stainless steel compositions with increased corrosive resistance



United States Patent 3,522,037 STAINLESS STEEL COMPOSITIONS WITH INCREASED CORROSIVE RESISTANCE Norbert D. Greene, Troy, N.Y., and Bryan E. Wilde, Livermore, Calif., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy No Drawing. Filed Oct. 31, 1966, Ser. No. 591,352

Int. Cl. (1221 39/20, 39/54 U.S. Cl. 75-125 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a modified 304 stainless steel having the following alloying elements 18 to 20% chromium, 8 to 12% nickel, up to .08% carbon, up to 1% silicon, up to 1% manganese, up to 0.1% sulfur, up to .45 phosphorous. The steel may also contain from .3 to .6% copper and/or .3 to .6% molybdenum.

This invention relates to a method of increasing appreciably the corrosion resistance of stainless steel by careful control of its composition and, in particular, to such a method as applied to 18 Cr-8 Ni stainless steel. The invention also relates to the product formed through the practice of such method.

Much effort has been expended on improving the corrosion resistance of austenitic steels by alloying and, as a result, numerous improved alloys have been developed. These alloy improvements are evident when the alloys spontaneously passivate upon immersion in a test solution, such as acid solutions containing ferric ions, and corrode at negligible rates.

Most of the eiforts directed toward improving corrosion resistance in austenitic steels, however, have been confined to alternations in the major components of the alloys, i.e., chromium and nickel, and to alloy additions, such as molybdenum, copper and titanium. In order to retain the advantages of the easy to roll and form 18 Cr-8 Ni stainless steel, the present efiort has been concentrated primarily in reducing the wide variation in corrosion resistance in moderately oxidizing environments (e.g., aerated sulphuric acid) of this steel.

The phenomenon of wide variation in corrosion resistance of 18 Cr-8 Ni stainless steel has been termed borderline passivity and has been attributed to variations in corrosive environments. conventionally, the problem of such 'wide variation is avoided by using stainless steel alloys, such as Types 310 and 316, which are more readily passivated in moderately oxidizing environments. In contrast, by carefully controlling the composition of Type 304 stainless steel, it has become possible through the present invention to combine the advantages of both Types 304 and 316 stainless steel, i.e., retaining the easy to roll and form characteristics of Type 304 and combining with these characteristics the good corrosion resistance of Type 316 to weakly oxidizing acid media. Type 304 retains its good corrosion resistance to moderately and strongly oxidizing acid media.

Accordingly, it is an object of the present invention to provide a modified Type 304 stainless steel with improved corrosion resistance.

It is another object of this invention to provide an improved Type 304 stainless steel wherein improved corrosion resistance is achieved through control of existing alloying and impurity eltments in lieu of the addition of elements.

A further object of this invention is to provide a method of improving the corrosion resistance of Type 304 stainless steel which is not restricted to a single composition but may be altered, depending on source material impurity.

A further object of this invention is to provide a method of improving the corrosion resistance of Type 304 stainless steel which avoids the detrimental influence of Mn and enhances the beneficial effect of C on such compositrons.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention.

Precision electrochemical and corrosion studies using substantially forty samples of commercially manufactured Type 304 stainless steel have demonstrated passivating tendencies in such steel which vary markedly from sample to sample, with the extremes varying by a factor of 250. In a few instances, the samples passivated as readily as the more expensive Type 316 alloy; however, the overwhelming majority of samples exhibited poor resistance to weakly oxidizing acid media in relation to such geosistance of other types of stainlesssteel, such as Type,

It has been established that corrosion resistance of a given metal, i.e., passivating tendency, may be increased by either reducing the critical anodic current density of the metal or shifting its primary passive potential to amore active potential. This can be accomplished by changing anodic dissolution behavior by alloying, and the amount of experimentation necessary to establish the trend for a given alloy addition is much smaller than it is for conventional corrosion testing. Since most of the commercially important metals and alloys possessing active-passive transitions have small exchange current densities for hydrogen evolution, passivation of'these metals occurs in media containing small quantities of oxidizers, such as oxygen or ferric ions. Under these conditions the reduction process is under diffusion control. It follows, therefore, that alloying additions which decrease the critical amodic current density, 1 rather than alter the position of the primary passive potential E are more effective in increasing passivating tendency. Multiple cor relation analysis of the samples tested has yielded the, following relationship between the critical anodic current density, 1,, and the sample composition:

I =15471+373 (Mn)+7600 (S)-750 (Mo)-6500 c 840 cm-1240 (Cu) where I is in microamperes/cm. and the alloy concentration is in weight percent.

The foregoing and the table, below, show that critical anodic current density is inversely related to passivating tendency.

TABLE [Corrosion Rates of Type 304 Stainless Steel Samples in 1 Normal Sulfuric Acld at 27 0. Exposed to Air--Hour Exposure] 1 Not caleulatedincomplete chemical anaylsis.

3 The alloying elements in austenitic stainless steels are listed below, together with the effect or function of each element:

Element Source Efiect or Function Chromium-.-" Intentional addition-.. Increases corrosion resistance to oxidizing acid environ ments.

Nickel Intentional addition-.- (1) Increases corrosion resisttance to oxidizers when present with Cr.

(2) Forms austenitic alloy (face-centered cubic) with excellent ductility.

Carbon Impurity from coke (1) Increases susceptibility in blast furnace. to intergranular attack if steel is incorrectly heat treated. Type 304L (0.03 max. was produced to avoid this problem, generally considered detrimental except in the present disclosure.

Sulfur Impurity from iron (1) Forms FeS in alloy which ore also present in causes brittleness (hot Cr and Ni additions. sltiortness) at high tempera ure.

(2) Decreases corrosion resistance in most media.

(3) It is universally considered detrimental.

Manganese Intentional addition; (1) Used to reduce detrimenalso enters as imtal mechanical eiiects of purity from slag. sulfur.

(2) Previously thought to have no effect on corrosion behavior.

Silicon Impurity resulting No significant effect in mefrom steel-making chanical behavior or corprocesses. Also rosion resistance when added to remove present in small amounts. oxygen.

Phosphorus Impurity in iron ore (1) Causes brittleness when and in Cr and Ni present in large amounts. additions. (2) Decreases corrosion resisttance of steel when present in large amounts.

Copper Impurity from cc (1) Improves corrosion resistcluded brass scrap. ance of steel and stainless steel.

(2) No efiect on mechanical properties when present in small quantities.

Molybdenum... Impurity in Type (1) Improves corrosion re- 30; (from scrap); sistance. intentional addition (2) Increases strength and in Type 316. makes working and rolling more diflicult.

Conventional compositions of Type 304 and Type 316 stainless steels are:

The usual Type 304 stainless steel 18-8 possesses the desirable characteristics of being easy to roll and form While having good corrosion resistance in moderate and strong oxidizing acid media (e.g. nitric acid). This type, however, has a distinct disadvantage in having poor resistance to weakly oxidizing acid media (e.g. air-free sulfuric acid).

In contrast, Type 316 stainless steel has better resist ance to weakly oxidizing media than Type 304 but is less resistant to strongly oxidizing media (eg, HNO more diflicult to roll and form and more expensive than Type 304. By carefully controlling the composition of Type 304 through minor variations in alloy and impurity content, it has become possible to combine the advantages of Types 304 and 316 stainless steel. The result of such controlled composition is a stainless steel that is easy to roll and form, has good corrosion resistance to weakly, moderately and strongly oxidizing acid media and is priced between Types 304 and 316. A typical alloy formed in accordance with the foregoing teachings would have the following composition, viz:

Of the alloying elements which influence corrosion behavior, Cr and Mn are the alloy additions, While S. Mo, C and Cu are the residual impurities. Therefore, depend ing on the basic purity of the alloy, the beneficial elements can be altered to achieve improved corrosion resistance.

Arbitrarily, critical anodic current density can be used as a measure of corrosion resistance as follows.

na./cm.

0-100 Excellent resistance (similar to Type 316 alloy). -500 Moderate resistance. 500 and above Inferior resistance.

Thus, it is possible to alter the chemical composition of Type 304 stainless steel a number of different ways to achieve superior corrosion resistance as indicated above. For example, an alloy, within standard specifications, with a composition of 19.0% chromium, 0.8% carbon, 0.25% manganese, 0.01% sulfur and free of copper and molybdenum'impurities would possess excellent resistance. Although the most desirable composition would contain small amounts of sulfur and manganese and high quantities of chromium, carbon, copper and molybdenum, this might not always be feasible in practice.

The following alloys represent Type 304 stainless steel in controlled compositions and modifications, made in accordance with this invention, which possess corrosion resistance equal to Type 316 stainless steel.

(1) Modified Type 304 stainless steel with improved corrosion resistance:

, (2 Controlled composition Type 304 stainless steel whichpo ssesses' improved corrosion resistance. This alloy contains no Cu or Mo, and is within specified Type 304 composition range:- e

7 Percent Cr 19.00-20.00. Ni 8.00-12.00. C 0.06-0.08 max. Si 1.00 max. Mn 0.25 max. S 0.010 max. P 0.045 max.

(3) Improved Type 304L (low carbon grade) stainless steel. This alloy is Within specified composition range for this type and possesses superior corrosion resistance:

Percent Cr 20.00 minimum. Ni 8.00-12.00.

C 0.03 max.

Si 1.00 max.

Mn 0.25 max.

S u 0.010 max.

P 0.045 max.

(4) Modified Type 304L with improved corrosion resistance:

Percent Cr 18.00-20.00. Ni 8.00-12.00. C 0.03 max.

Si 1.00 max. Mn 0.80 max.

S 0.010 max. P 0.045 max.

Mo 0.3-0.5 max.

Cu 0.3-0.5 max.

(5) Modified Type 304 stainless steel manufactured from high sulfur ore and scrap with improved corrosion resistance:

(6) Copper-containing Type 304 for improved corrosion resistance:

Percent Cr 19.00-20.00. Ni 8.00-12.00. C 0.06-008 max. Si 1.00 max. Mn 0.25 max.

S 0.010 max. P 0.045 max. Cu 0.4-0.6.

(7) Molybdenum-containing Type 304 for improved corrosion resistance:

Percent Cr 19.00-20.00. Ni 8.00-12.00-. C 0.06-0.08. Si 1.00 max. Mn 0.25 max. 5 0.010 max. P 0.045 max. Mo O.4-0.6.

It thus has been found possible to predict and control the corrosion resistance of Type 304 stainless steel by minor variations in alloy and impurity content. Under the most favorable conditions, it is possible to produce Type 304 alloy with a corrosion resistance equal that of Type 316 stainless steel.

The method of this invention is applicable to moderately oxidizing acid media such as aerated sulfuric acid. It should be applicable to all similar environments at low and high temperatures. Examples are phosphoric, hydrofluoric, perchlorin, acetic and other organic acids containing air and ferric, ceric, nitrate, chromate and other oxidizing salts. Alloy differences should not be noticeable in strong oxidizing media such as nitric acid. In such electrolytes the difierences in alloy or passivating tendency become negligible. In addition, the present method offers a substitute for more expensive steels (Types 309, 310, 316, 317) in moderately oxidizing media.

Obviously many modifications and variations of the present application are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A modified Type 304 stainless steel having improved corrosion resistance consisting essentially of from 19.0% to 20.0% chromium, from 8.0% to 12.0% nickel, up to 0.08% carbon, up to 1.0% silicon, up to 0.30% manganese, up to 0.01% sulfur, up to 0.045% phosphorus, from 0.30% to 0.50% molybdenum, from 0.30% to 0.50% copper and the balance substantially all iron, the steel being characterized by exhibiting good resistance to corrosion in strong, moderate and weak oxidizing acid media in combination with good ductility.

2. The composition of claim 1 wherein said carbon content is from 0.06% to 0.08%.

3. A controlled composition Type 304 stainless steel possessing improved corrosion resistance consisting essentially of from 19.0% to 20.0% chromium, from 8.0% to 12.0% nickel, from 0.06% to 0.08% carbon, up to 1.0% silicon, up to 0.25% manganese, up to 0.01% sulfur, up to 0.045% phosphorus and the balance substantially all iron, said steel characterized -by exhibiting good resistance to corrosion in strong, moderate and Weak oxidizing acid media in combination with good ductility.

4. A modified Type 304 stainless steel having improved corrosion resistance consisting essentially of from 18.0% to 20% chromium, from 8.0% to 12.0% nickel, up to 0.03% carbon, up to 1.0% silicon, up to 0.80% manganese, up to 0.01% sulfur, up to 0.045% phosphorus, from 0.30% to 0.50% molybdenum, from 0.30% to 0.50% copper and the balance substantially all iron, said steel having good resistance to corrosion in strong, moderate and weak oxidizing acid media in combination with good ductility.

5. A modified Type 304 stainless steel having good ductility in combination with properties of Type 304 stainless steel yet good resistance to corrosion in strong, moderate and weak oxidizing acid media, said modified steel consisting essentially of from 19.0% to 20.0% chromium, from 8.0% to 12.0% nickel, up to 0.08% carbon, up to 1.0% silicon, up to 0.25% manganese, up to 0.01% sulfur, up to 0.045% phosphorus, from 0.40% to 0.60% copper and the balance substantially all iron.

6. The composition of claim 7 wherein said carbon content is from 0.06% to 0.08%.

7. A modified Type 304 stainless steel having good resistance to corrosion in weak, moderate and strong oxidizing acid media in combination with good ductility, said modified steel consisting essentially of from 19.0% to 20.0% chromium, from 8.0% to 12.0% nickel, from 0.06% to 0.08% carbon, up to 1.0% silicon, up to 0.25% manganese, up to 0.01% sulfur, up to 0.045% phosphorus,

7 8 from 0.4% to 0.6% molybdenum and the balance sub- 3,152,934 10/1964 Lula 75--128 X stantially all iron. ,1 2,041 6/1965 Kanter 75-128 X References Cited 3,258,370 6/1966 Floreen 75128 X UNITED STATES PATENTS HYLAND BIZOT, Primary Examiner Re. 26,454 9/1968 GolIer 7s 12s X 5 1,841,752 1/1932 McKee 75-128 US. Cl. X.R. 2,267,866 12/1941 Kaye 75-128.9 75128 

